The invention relates to a gas spring having speed regulation. Various forms of gas spring are known and may be used, for example, for raising or opening doors, such as hatchback doors, and lids in motor vehicle bodies for example, though they may be used in many other applications. In such springs, a cylinder is at least partially filled with gas which tends to expel a piston rod. It is desirable to be able to regulate the speed of movement of the piston rod so as to be able to achieve a desired speed of operation but with appropriate deceleration at the end of the travel of the piston rod.